I opened the heavy door and went inside.
A new door has been opened (a new opportunity has arisen).
Specifically the title page of a book or publication. Less common than the 'door' sense.
The author's name is written on the title page of the book.
He opened the car door and got in.
It's cold, so please close the door.
The door opened automatically.
That discovery opened the door to a new era.
I locked the door before going out.
front door (cabinet, bus, etc.) · half-title; bastard title
external door; outer door